Wheel slip control systems are employed most commonly on articulated highway vehicles to shorten stopping distance and to permit directional control of the vehicle even in panic stops. Wheel slip control systems usually operate by sensing conditions of speed and/or acceleration which are normally indicative of an existing or incipient skid condition. Upon having detected a skid condition, the wheel slip control system operates one or more electric solenoids in the vehicle brake line which modify the fluid pressure being applied to the brakes. The modification of fluid pressure is a temporary reduction in the fluid pressure applied to the brakes in order to permit the skidding wheel to spin up to approximately vehicle speed.
Wheel slip control systems, such as those disclosed in U.S. Pat. Nos. 3,840,816; 3,988,599 and 3,842,355, incorporated herein by reference, employ wheel speed sensors which generate an alternating signal whose frequency varies in proportion to the wheel rotational speed. A wheel speed signal processor converts the variable-frequency wheel speed signal into a dc velocity signal whose amplitude is proportional to the frequency of the wheel speed signal. Logic circuits decide whether the characteristics of the velocity signal, in some cases including the acceleration content, are characteristic of a wheel skid and, if so, generate an electrical output signal which energizes one or more solenoids to provide relief of braking pressure.
There are a number of malfunction conditions afflicting wheel slip control systems which may interfere with proper operation of the braking system or may permanently damage the wheel slip control system. One such malfunction condition is the long-term energization of a solenoid valve rather than its normal intermittent actuation. Since the solenoid valve reduces braking pressure, its long-term energization due to system faults can excessively reduce the desired application of braking force and can vent excessive quantities of fluid pressure sometimes exceeding the capacity of the pressure source. U.S. Pat. Nos. 3,842,355 and 3,988,599 disclose a timer which received the energization signal of one or more solenoids. If the solenoids remain continuously energized longer than a predetermined time, the timer triggers a crowbar circuit which blows a system fuse to disable the wheel slip control system and return control of braking to its normal manual mode.
Certain types of wheel speed sensors have a malfunction mode which produces a cyclic variation in output frequency when no variation in wheel speed in fact exists. One such malfunction exhibits a complete loss of signal over a portion of the revolution of the wheel. This may occur, for example by axial misalignment of the rotating element of the wheel speed sensor which causes the space between the fixed and rotating members to cyclically move into and out of effective working range. When the members move out of working range, the wheel speed signal suddenly changes to zero and then resumes after a short delay.
The logic circuit interprets this abrupt change in wheel speed signal as a large deceleration which signifies a skid and applies an energization signal to the solenoid. The abrupt resumption of the wheel speed signal is interpreted by the logic circuit as the end of the skid. The logic circuit thereupon deenergizes the solenoid.
This type of malfunction typically occurs at least once per wheel revolution and usually persists for a short time per cycle. Cyclic energization of the solenoid by this type of signal not only vents excessive quantities of fluid pressure but may also rapidly lead to destruction of the solenoid.
U.S. Pat. No. 4,036,537 attempts to solve the problem of the spurious cyclic sensor signal by gating the cyclically varying velocity-derived signal with a second signal generated in the logic circuit from a combination of signals relatively immune to velocity signal dropouts. The gate blocks the velocity-derived signal from the solenoid until the first occurrence of the second signal whereupon a timer enables the gate for a predetermined period thus permitting actuation of the solenoid by the velocity-derived signal even though it may contain sensor noise. The second signal may be of the type disclosed in U.S. Pat. No. 3,988,599.
A different approach to solving the problem of cyclic sensor fallout was disclosed in U.S. Pat. No. 4,013,324. In this patent, a timer is started by the onset of energization of a solenoid. The termination of energization is normally operative to reset the timer. A short-pulse discriminator is effective to prevent solenoid energization pulses shorter than a predetermined length from being effective to reset the timer at their termination. The timer is thereby permitted to complete its timing cycle and generate a disabling signal.